1. Field of the Invention
The present invention relates to a display method and a display apparatus using this method, and particularly to a display method related to gray-level representation and a display apparatus using this method.
2. Description of the Background Art
In recent years, the input image signal to a matrix-type image display apparatus (which is hereinafter also referred to simply as a display apparatus) has an increased number of bits because of improved data processing ability and the like, and the number of bits of the input image signal may exceed the number of bits that the data driver IC can output. That is, the number of representable gray levels of a display apparatus depends on the performance of the data driver IC, and phenomena like “missing of gradations” and “slipping of tone” occur when the number of bits that the data driver IC can output is smaller than the number of bits of the input image signal. When phenomena like missing of gradations and slipping of tone occur, the display apparatus cannot faithfully reproduce images as the user intends.
Then, faithfully reproducing images in the display apparatus requires changing the data driver IC so that it can output a larger number of bits than the input image signal. However, configuring the data driver IC so that it can output an increased number of bits involves increased costs for the data driver IC. Accordingly, for example, the FRC (Frame Rate Control) may be adopted so that the gray levels represented by the number of bits of the input image signal can be expressed even when the data driver IC is only capable of outputting a smaller number of bits than the number of bits of the input image signal.
In the FRC, for display of one pixel dot in m frames (one period), a gray level Gp is displayed in n frames (n<m), and a gray level Gq is displayed in the remaining (m−n) frames, so that the viewer's eyes recognize a gray level (a pseudo gray level) that corresponds to a weighted time mean based on the ratio of frames of the gray level Gp and the gray level Gq. Specifically, a display apparatus adopting the FRC is disclosed in Japanese Patent Application Laid-Open No. 10-49108 (1998), for example.
However, when a j-bit input image signal is displayed with a data driver IC that is capable of outputting i bits (i<j), the number of representable gray levels is still insufficient even when the display apparatus adopts FRC. Specifically, when {2^(j−i)−1} pseudo gray levels are generated by FRC between individual i-bit gray levels by handling 2^(j−i) frames as one set, the display apparatus is then capable of outputting {2^j−2^(j−i)+1} gray levels. However, the number of gray levels is still fewer by 2^(j−i)−1 than the number of gray levels (2^j) of the input image signal to be displayed. The lack of gray levels results in so-called “missing of gradations” in displayed images.
To solve the missing of gradations and obtain the absent gray levels, it is necessary to apply the frame rate control (FRC) with a different number of frames from 2^(j−i) only between certain gray levels Gr and Gr+1. Specifically, in a method for remedying the lack of gray levels, {2^(j−i+1)−1}−1 pseudo gray levels are generated by handling {2^(j−i+1)−1} frames as one set, where {2^(j−i+1)−1} is the sum of 2^(j−i) and {2^(j−i)−1} that is equal to the number of absent gray levels.
However, in this conventional display method, while pseudo gray levels between some gray levels are generated by handling 2^(j−i) frames as one set, pseudo gray levels between other gray levels are generated by handling {2^(j−i+1)−1} frames as one set, in order to remedy the lack of gray levels. Accordingly, when an image changes in time between the two pseudo gray level ranges, the frequency becomes extremely smaller when pseudo gray levels with {2^(j−i+1)−1} frames are displayed, than when pseudo gray levels with 2^(j−i) frames are displayed, and it is likely to be recognized as flicker by human eyes.